- Email: [email protected]
Energy selective contacts for hot carrier solar cells
ARTICLE IN PRESS Solar Energy Materials & Solar Cells 94 (2010) 1546–1550
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Energy selective contacts for hot carrier solar cells Santosh K. Shrestha , Pasquale Aliberti, Gavin J. Conibeer ARC Photovoltaic Centre of Excellence, University of New South Wales, Sydney, NSW 2052, Australia
a r t i c l e in f o
a b s t r a c t
Article history: Received 29 June 2009 Received in revised form 22 October 2009 Accepted 26 November 2009 Available online 15 January 2010
Double barrier resonant tunnelling structures consisting of silicon quantum dots (QDs) in silicon dioxide (SiO2 ) matrix have been studied for Energy Selective Contacts for Hot Carrier solar cell. A single layer of silicon QDs has been fabricated by high temperature annealing of SiO2 / Si-rich oxide (SRO)/SiO2 layers deposited by RF magnetron sputtering. Compositional analysis of SRO films obtained with different sputtering target has been accurately measured with Rutherford backscattering spectroscopy. Size-controlled growth of Si QDs has been studied with photoluminescence measurements which demonstrate that QD sizes can be controlled with SRO layer thickness. In addition, resonant tunnelling behaviour of SiO2 / Si QD/SiO2 structures has been investigated. & 2009 Elsevier B.V. All rights reserved.
Keywords: Hot Carrier solar cell Energy Selective Contacts Quantum dot fabrication Third generation photovoltaics Resonant tunnelling
1. Introduction Efficiency of solar cells has significantly improved over the last few decades. However, realised values are much lower than the theoretical limits. Several technologies are being investigated to enhance the efficiency of photovoltaic systems. The Hot Carrier (HC) solar cell is a promising third generation photovoltaic devices. It aims to tackle a major loss in a convectional solar cells due to the thermalisation of photoexcited carriers with the lattice [1–4]. The efficiency of the HC solar cell is predicted to be over 65% for non-concentrated solar radiation. The concept of HC solar cell is illustrated in Fig. 1. It consists of an absorber which can significantly reduce thermalisation of photoexcited hot carriers, from few picoseconds for most semiconductors to few nanoseconds. This is to allow sufficient time for the collection of the hot carriers. Another important requirement for the realisation of HC solar cell is Energy Selective Contacts (ESCs). These ESCs, like an energy filter, only allow carrier within a narrow energy range to pass through to the metal contacts; carriers with other energies are reflected back into the absorber where they are renormalised by carrier–carrier scattering. Thus, only a small fraction of their excess energy above the band edge is lost when the hot carriers come in contact with the cold carriers in the metal electrodes [5]. Double barrier structures consisting of Si QDs in a dielectric matrix are a potential candidate for ESCs, with QDs providing a discrete energy level between two insulating barriers [5]. This is
Corresponding author. Tel.: + 61 2 93854454; fax: +61 2 93855412.
E-mail address: [email protected] (S.K. Shrestha). 0927-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2009.11.029
expected to give conduction of carriers strongly peaked at the discrete energy levels, and lower at other energies. Such a structure should exhibit negative differential resistance (NDR) characteristics in DC I–V measurement. The use of quantum dots is also advantageous since they offer total energy selection rather than confinement in one dimension such as by quantum wells. However, the fabrication of quantum dots of uniform size, for the better selectivity of hot carriers, is very challenging. For practical HC solar cell it is also necessary to be able to tune the resonance energy in order to optimise the carrier collection. This may be achieved by controlling the size of the quantum dots in which discrete energy is modified due to quantum confinement. In this paper we investigate fabrication of size-controlled growth of Si QDs in SiO2 for Energy Selective Contacts. The structures have been deposited by RF magnetron sputtering. The stoichiometry of the films obtained with different sputtering targets have been investigated with Rutherford backscattering spectroscopy (RBS). The size of the QDs and its correlation with the thickness of silicon-rich oxide layer, SRO (SiOx ; 0 ox o2), have been studied with photoluminescence (PL) studies. Results of I–V measurements on selected double barrier structures consisting Si QDs are also discussed.
2. Sample preparation Double barrier resonant tunnelling structures were grown by depositing alternate layers of SiO2 , SRO and SiO2 films with a single target RF-magnetron sputtering system. A combined sputtering target consisting of 4-in quartz disc partially masked with a patterned silicon wafer was used to deposit the layers.
ARTICLE IN PRESS S.K. Shrestha et al. / Solar Energy Materials & Solar Cells 94 (2010) 1546–1550
1547
Fig. 1. Schematic diagram of a hot carrier solar cell concept. The hot carriers in the absorber are collected through the energy selective contacts before they thermalise in the lattice. Adapted from [8].
Before the growth, the chamber was pumped down to a base pressure of, typically, less than 2:5 10-3 mtorr. SRO layers were deposited with argon sputtering; additional O2 was supplied during the deposition of SiO2 layers. The chamber pressures were 0.75 and 1.5 mtorr for the SRO and SiO2 depositions, respectively. Samples were grown without substrate heating and the RF power of 25 W was used. Under these conditions the growth rate was about 1 nm/min for both the SRO and SiO2 films which was determined by Dektak measurements by profiling areas with and without film. Samples were grown both on quartz and highly doped silicon substrates to facilitate optical and electrical characterisations, respectively. Substrates were cleaned with standard RCA1 and RCA2 solutions prior to the film deposition. Since samples on quartz and silicon were grown under same conditions, their compositional, optical and electrical properties are expected to be similar. Following the growth, the samples were typically annealed for 2 h at 1100 3 C in high purity nitrogen (99.9999). During the high temperature annealing, the excess Si in the SRO layer segregates to form QDs between the two oxide layers. The formation of QDs can be described with the following equation: SiOx -
x x SiO2 þ 1- Si 2 2
ð1Þ
This method of self-organised growth of Si quantum dots was first reported by Zacharias et al. [6] and has been developed at UNSW over the last few years using RF sputtering [7]. The size of the QDs is limited by the thickness of the SRO layer between the two barrier layers. Eq. (1) shows the dependence of density of quantum dots on the composition of the SRO layer. The density of the dots is higher if the SRO layer has a lower x, i.e., smaller O/Si ratio [6]. Several samples were deposited using different combinations of Si and quartz sputter targets to vary composition of SRO layers. In order to investigate size-controlled growth of Si QDs, several SiO2 =SRO=SiO2 structures with different SRO thicknesses were also grown using the same sputtering target. To facilitate comparison among different samples, oxide layer thicknesses were kept constant. Fig. 2 shows plan-view TEM image of a typical SiO2 =SRO=SiO2 structure grown on a silicon substrate after annealing at 11003 for 2 h. In this case a combined Si and quartz sputtering target with 33% silicon-excess, corresponding to oxygen to silicon ratio ¼ 1, was used. Silicon QDs in the oxide matrix are clearly visible. The size of the QDs is of about 5 nm which is similar to the thickness of the SRO layer, although slight variation in the QD sizes is evident.
Fig. 2. Typical TEM image of a SRO layer in SiO2 matrix after annealing at 1100 3 C. Si QDs are clearly visible.
Table 1 Oxygen to silicon ratio of silicon-rich oxide films as determined from sputter targets and Rutherford backscattering spectroscopy. Sample
ðO=SiÞTarget
ðO=SiÞRBS
A B C D
1.29 1.10 1.00 0.77
1.33 1.05 1.03 0.78
3. Results and discussion 3.1. Compositional analysis Rutherford backscattering spectroscopy (RBS) is a well established technique for the quantitative compositional depth profiling of thin films [9]. Thus, this technique has been used to investigate stoichiometry of SRO films obtained with different sputtering targets. SRO films were deposited using four different combined Si and quartz sputtering targets. The expected oxygento-silicon ratio, O/Si, for these films, as derived from the sputtering rates, are given in Table 1. RBS measurements were performed at the Australian National University, Canberra using 2 MeV 4He ion beam delivered by the 1.7 MV NEC Tandem accelerator. Projectile ions were detected at a scattering angle of y ¼ 1683 . Fig. 3 shows a measured energy spectrum from RBS measurement for a SRO film grown on a silicon substrate using 53% silicon-excess target (sample D). For each element, the high energy ions are from the surface of the sample, while the low energy ions are from deep inside the film. The silicon and oxygen from the film are clearly evident. The oxygen yield of the film, YO , was obtained by subtracting the oxygen signal from the background counts as shown in Fig. 3(b). The determination of silicon yield, YSi , was complicated since silicon was present both in the film and the substrate. The silicon in the film, in this case, was obtained by fitting the energy spectra with a combination of the linear and the error functions, as shown in Fig. 3(c). Here, the dashed-dot curve is the overall fit to the experimental data and the dashed curve is the contribution due to the substrate. The
ARTICLE IN PRESS 1548
S.K. Shrestha et al. / Solar Energy Materials & Solar Cells 94 (2010) 1546–1550
Fig. 3. (a) RBS energy spectrum from a typical SRO film. Fits to determine (b) oxygen and (c) silicon content of SRO film are shown.
solid curve shows the contribution due to the silicon from the film. It is evident from Fig. 3(b) and (c) that the quality of fits is very good. The quality of fit was similar for other samples. In order to compare the films from different growths, oxygen to silicon ratio of SRO films has been determined as follows: O YO YSi -1 ð2Þ ¼ dsO dsSi Si where dsO and dsSi are scattering cross section for oxygen and silicon, respectively. Results of RBS analysis are given in Table 1. The statistical uncertainty of the measurements was less than 1%. However, uncertainty of O/Si ratio derived from the fit may have been as high as 3% for few measurements. RBS measurements show that SiO2 is stoichiometric within the experimental uncertainties. Importantly, O/Si ratio of SRO films determined with RBS correlates well with that calculated from the sputtering targets. The discrepancy may be due to the uncertainty in the estimation of O/Si ratio from the sputtered targets, primarily due to different sputter rates for silicon and quartz targets. 3.2. Photoluminescence studies For the investigation of size-controlled growth of QDs, several samples consisting of a single layer Si QDs in SiO2 matrix were prepared by high temperature annealing of SiO2 =SRO=SiO2 layers. The SRO layer thicknesses were in the range of 1.8–7 nm. Thicknesses of the oxide layers were about 6 nm for all the samples, except for the two samples with SRO layer thickness of 1.8 nm and 2.4 nm. For these samples a 30 nm capping oxide layer was deposited to prevent possible oxidation of QD layer during
Fig. 4. (a) Results of PL measurement on a series of samples with a single layer of Si QDs in SiO2 matrix. (b) PL peak energy as a function of QD diameter. The solid curve is the best fit of PL energy with inverse square of QD diameter. For comparison data from selected literature are shown.
the high temperature annealing. For comparison, all the structures were grown with the same sputtering target (in this case 33% silicon-excess target with O=Si ¼ 1) and under similar growth conditions. Fig. 4(a) shows results of room temperature PL measurements on several samples. The 532 nm (2.34 eV) line of He-Cd laser with 50 mW power was used as the excitation source. PL signals were collected using a co-focal-lens system and detected with a double grating monochromator and an array of silicon photodetectors. The data were corrected for errors introduced by the collection system and spectral response of the detector array according to [10]. The normalised PL spectra from different samples are shown in the diagram. It should be noted that no distinct PL signature was observed from as-deposited samples. The signal-to-noise ratio is quite good, although PL was measured on a single layer of Si QDs, demonstrating a strong absorption coefficient of these QDs. This ratio is similar for the samples with SRO layer thickness greater than 3 nm, however, it is relatively weaker for the sample with smaller SRO layer thicknesses. Strong decrease in PL intensity from samples containing smaller nanocrystals are reported in literature which are attributed to decrease of the density of nanocrystals and the smaller absorption cross section [6,11]. These could also explain relatively weaker PL signal from our samples containing smaller QDs. The full width at half maximum of the PL peaks are quite broad ( 4 0:25 eV) suggesting a large size distribution of QDs in the samples. This is consistent with the TEM image where a slight variation in QD sizes has been observed (see Fig. 2). Large distributions of size of nanocrystals in multilayered structure or bulk film have also been reported, for example see [6,11]. However, PL peaks from different samples are clearly separated
ARTICLE IN PRESS S.K. Shrestha et al. / Solar Energy Materials & Solar Cells 94 (2010) 1546–1550
which demonstrates that average sizes of QDs are different for different samples. Importantly, a blue-shift of the PL peak is observed with the decrease in the SRO layer thickness. The shift is larger for the smaller QDs whereas it is smaller for the bigger QDs which are consistent with the quantum confinement model [12]. This is a significant result which shows that size of the quantum dots can be controlled with the SRO layer thickness. Fig. 4(b) shows the PL peak energy, derived from Fig. 4(a), as a function of the respective QD sizes. Here average QD size has been estimated to be equal to the SRO thicknesses, which is a reasonable estimation as the size of QDs is limited by the SRO layer thickness as discussed in Section 2. A non-linear decrease of PL energy with increasing QD sizes is evident. The decrease is rapid for the smaller QDs but slower for larger dots. The solid curve is the best fit of PL peak energy with inverse square of QD diameter which is in good agreement with the data. For comparison, data from selected literature [6,13,14] are shown which are in reasonable agreement with our data. Our data are also consistent with data for Si QDs in multilayered structure (SiO2 =Si QDs=SiO2 = . . . Si QDs=SiO2 ) and grown with similar technique [15]. This may indicate Si QDs in single and multilayered structures have similar optical properties.
3.3. Electrical measurements For I–V measurements to investigate resonant tunnelling through the double barrier structures discussed earlier, devices as schematically shown in the inset of Fig. 5 were fabricated using 33% silicon-excess target. This consisted of a single layer of Si QDs in SiO2 grown on a heavily doped Si substrate. A silicon capping layer of about 50 nm thickness was deposited with a highly doped silicon target. The thicknesses of the SRO and SiO2 layers were approximately 5 nm. A shadow mask was used during the growth to obtain isolated devices. The samples were annealed at 1100 3 C in nitrogen for 2 h during which QDs are formed. Aluminium contacts were deposited by evaporation using a shadow mask for electrical measurements. Fig. 5 shows results of room temperature I–V measurements on a typical device. Size of the mesa for this sample was about 5 mm2 . Here, the open and filled data points are the results of two subsequent measurements. In the both I–V profiles, an indication
1549
of negative differential resistance (NDR) can be observed around 1.3 V. Jiang et al. have described a model for resonant tunnelling through QDs in double barrier structure [16]. This model can, in principle, be applied to study the correlation of NDR peaks with resonant energy levels. However, more data would be required for reliable investigation. The observed NDR characteristics show increased conduction for the carriers in the energy range ( 1:321:5 eV) compared to the carriers outside this range. However, the observed NDR peak is very broad, which demonstrates a poor energy selectivity. Ideally hot carriers need to be extracted at a discrete energy through mono-energetic contacts for optimum efficiency [2]. However, in this case the power output from the device would be minimum. In a real device, however, width of the ESC, dE 40 which means that entropy will be generated on carrier cooling and thus efficiency will be reduced. Hence, dE should be kept as small as possible. A rough estimate of dE 25 meV has been recommended as a reasonable value for preliminary contact [5]. However, the observed half width of the NDR peak is more than 400 meV, which is an order of magnitude larger than the value suggested above. Hence, significant improvement in the quality of resonance is needed for these structures to be applicable as Energy Selective Contact for Hot Carrier solar cells. Nevertheless, such a result at room temperature is encouraging and demonstrates proof-of-concept. The broad NDR peak can be attributed to transport through QDs with slightly varying sizes around a mean diameter which will result in an overlap of resonant energy levels. This will be consistent with the broad PL peaks observed for all the samples suggesting a distribution of QD sizes in the structure. In addition, broadening of the NDR peak could be due to the deviation of QDs from the middle of the matrix which is expected to decrease the transmission probability significantly. It is also clear that the I–V profile has changed during the two subsequent measurements which indicate presence of defects in the material. Understanding of different types of defects in the SiO2 matrix and at the interfaces of QDs and SiO2 is a subject of further study. It should also be noted that device breakdown occurs if voltage in the access of, typically, 2 V is applied. The oxide strength of these samples is estimated to be about 1.3 MV/ cm which is much smaller than that for the thermal oxide. This may be primarily due to the presence of Si QDs and inferior quality of sputtered oxide. Further work is in progress to improve the quality of material, especially the oxide quality, for example, by growth at elevated temperature.
4. Conclusion
Fig. 5. Results of two consecutive I–V measurements at room temperature on a typical resonant tunnelling device are shown. The open and filled data points represent the first and the second measurements, respectively. Structure of the device is schematically shown in the inset.
Double barrier structures consisting of a single layer of Si QDs in SiO2 matrix have been studied for Energy Selective Contacts required for Hot Carrier solar cells. These structures were grown with RF magnetron sputtering using Si and quartz targets followed by high temperature annealing. The formation of quantum dots has been confirmed by transmission electron microscopy. Compositions of SRO films obtained with different sputtering targets have been accurately measured with Rutherford backscattering spectroscopy. Measurements show that O/Si ratio of SRO films correlates with the sputtering targets thus can be controlled. All the samples have shown good PL signals although they contained only a single layer of Si QDs. A blueshift of the PL peak is observed with the decrease in the SRO layer thickness which is consistent with quantum confinement model. Importantly, PL studies have demonstrated that the size of QDs can be controlled very well by the thickness of silicon-rich oxide layer, in between two oxide layers. I–V measurements have
ARTICLE IN PRESS 1550
S.K. Shrestha et al. / Solar Energy Materials & Solar Cells 94 (2010) 1546–1550
shown an evidence of NDR, which is a characteristic of resonant tunnelling, with some repeatability. However, the NDR peak is very broad suggesting a poor energy selectivity. Further work is focused on improving the quality of the material and position and quality of the resonance peak.
Acknowledgements The Photovoltaic Centre of Excellence is supported by the Australian Research Council. The authors would like to thank Shujuan Huang for the TEM images and Prof. Rob Elliman for the access to the RBS facility. References [1] R.T. Ross, A.J. Nozik, Efficiency of hot-carrier solar energy converters, J. Appl. Phys. 53 (1982) 3813–3818. ¨ [2] P. Wurfel, Solar energy conversion with hot electrons from impact ionisation, Sol. Energy Mater. Sol. Cells 46 (1997) 43–52. [3] M.A. Green, Third Generation Photovoltaics, Springer, Berlin, 2003. [4] Y. Takeda, et al., Hot carrier solar cells operating under practical conditions, J. Appl. Phys. 105 (2009) 074905:1–10.
[5] G.J. Conibeer, et al., Selective energy contacts for potential application to hot carrier PV cells, in: Proceedings of the 3rd World Conference on Photovoltaic Energy Conversion, Japan, 2003 pp. 2730–2733. [6] M. Zacharias, J. Heitmann, R. Scholz, U. Kahler, M. Schmidt, Size-controlled highly luminescent silicon nanocrystals: a SiO=SiO2 superlattice approach, J. Blasing Appl. Phys. Lett. 80 (2002) 661–663. [7] E.-C. Cho, et al., in: Proceedings of the 19th European Photovoltaic Solar Energy Conference, France, 2004, pp. 235–238. ¨ [8] P. Wurfel, A.S. Brown, T.E. Humphrey, M.A. Green, Particle conservation in the hot-carrier solar cell, Prog. Photovolt. Res. Appl. 13 (2005) 277–285. [9] L.C. Feldman, J.W. Mayer, Fundamentals of Surface and Thin Film Analysis, North-Holland, New York, 1986. [10] T.H. Gfroerer, Photoluminescence in analysis of surfaces and interfaces, Encyclopedia of Analytical Chemistry, 2000, pp. 9209–9231. [11] U. Kahler, H. Hofmeister, Visible light emission from Si nanocrystalline composites via reactive evaporation of SiO, Opt. Mater. 17 (2001) 83–86. [12] M.V. Wolkin, et al., Electronic states and luminescence in porous silicon quantum dots: the role of oxygen, Phys. Rev. Lett. 82 (1999) 197–200. [13] H. Takegi, et al., Quantum size effects on photoluminescence in ultrafine Si particles, Appl. Phys. Lett. 56 (1990) 2379–2380. [14] S. Takeoka, M. Fujii, S. Hayashi, Size-dependent photoluminescence from surface-oxidized Si nanocrystals in a weak confinement regime, Phys. Rev. B 62 (2000) 16820–16825. [15] G.J. Conibeer, et al., Silicon quantum dot nanostructures for tandem photovoltaic cells, Thin Solid Films 516 (2008) 6748–6756. [16] C.-W. Jiang, et al., Resonant tunneling through defects in an insulator: modeling and solar cell applications, J. Appl. Phys. 96 (2004) 5006–5012.
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Energy selective contacts for hot carrier solar cells Santosh K. Shrestha , Pasquale Aliberti, Gavin J. Conibeer ARC Photovoltaic Centre of Excellence, University of New South Wales, Sydney, NSW 2052, Australia
a r t i c l e in f o
a b s t r a c t
Article history: Received 29 June 2009 Received in revised form 22 October 2009 Accepted 26 November 2009 Available online 15 January 2010
Double barrier resonant tunnelling structures consisting of silicon quantum dots (QDs) in silicon dioxide (SiO2 ) matrix have been studied for Energy Selective Contacts for Hot Carrier solar cell. A single layer of silicon QDs has been fabricated by high temperature annealing of SiO2 / Si-rich oxide (SRO)/SiO2 layers deposited by RF magnetron sputtering. Compositional analysis of SRO films obtained with different sputtering target has been accurately measured with Rutherford backscattering spectroscopy. Size-controlled growth of Si QDs has been studied with photoluminescence measurements which demonstrate that QD sizes can be controlled with SRO layer thickness. In addition, resonant tunnelling behaviour of SiO2 / Si QD/SiO2 structures has been investigated. & 2009 Elsevier B.V. All rights reserved.
Keywords: Hot Carrier solar cell Energy Selective Contacts Quantum dot fabrication Third generation photovoltaics Resonant tunnelling
1. Introduction Efficiency of solar cells has significantly improved over the last few decades. However, realised values are much lower than the theoretical limits. Several technologies are being investigated to enhance the efficiency of photovoltaic systems. The Hot Carrier (HC) solar cell is a promising third generation photovoltaic devices. It aims to tackle a major loss in a convectional solar cells due to the thermalisation of photoexcited carriers with the lattice [1–4]. The efficiency of the HC solar cell is predicted to be over 65% for non-concentrated solar radiation. The concept of HC solar cell is illustrated in Fig. 1. It consists of an absorber which can significantly reduce thermalisation of photoexcited hot carriers, from few picoseconds for most semiconductors to few nanoseconds. This is to allow sufficient time for the collection of the hot carriers. Another important requirement for the realisation of HC solar cell is Energy Selective Contacts (ESCs). These ESCs, like an energy filter, only allow carrier within a narrow energy range to pass through to the metal contacts; carriers with other energies are reflected back into the absorber where they are renormalised by carrier–carrier scattering. Thus, only a small fraction of their excess energy above the band edge is lost when the hot carriers come in contact with the cold carriers in the metal electrodes [5]. Double barrier structures consisting of Si QDs in a dielectric matrix are a potential candidate for ESCs, with QDs providing a discrete energy level between two insulating barriers [5]. This is
Corresponding author. Tel.: + 61 2 93854454; fax: +61 2 93855412.
E-mail address: [email protected] (S.K. Shrestha). 0927-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2009.11.029
expected to give conduction of carriers strongly peaked at the discrete energy levels, and lower at other energies. Such a structure should exhibit negative differential resistance (NDR) characteristics in DC I–V measurement. The use of quantum dots is also advantageous since they offer total energy selection rather than confinement in one dimension such as by quantum wells. However, the fabrication of quantum dots of uniform size, for the better selectivity of hot carriers, is very challenging. For practical HC solar cell it is also necessary to be able to tune the resonance energy in order to optimise the carrier collection. This may be achieved by controlling the size of the quantum dots in which discrete energy is modified due to quantum confinement. In this paper we investigate fabrication of size-controlled growth of Si QDs in SiO2 for Energy Selective Contacts. The structures have been deposited by RF magnetron sputtering. The stoichiometry of the films obtained with different sputtering targets have been investigated with Rutherford backscattering spectroscopy (RBS). The size of the QDs and its correlation with the thickness of silicon-rich oxide layer, SRO (SiOx ; 0 ox o2), have been studied with photoluminescence (PL) studies. Results of I–V measurements on selected double barrier structures consisting Si QDs are also discussed.
2. Sample preparation Double barrier resonant tunnelling structures were grown by depositing alternate layers of SiO2 , SRO and SiO2 films with a single target RF-magnetron sputtering system. A combined sputtering target consisting of 4-in quartz disc partially masked with a patterned silicon wafer was used to deposit the layers.
ARTICLE IN PRESS S.K. Shrestha et al. / Solar Energy Materials & Solar Cells 94 (2010) 1546–1550
1547
Fig. 1. Schematic diagram of a hot carrier solar cell concept. The hot carriers in the absorber are collected through the energy selective contacts before they thermalise in the lattice. Adapted from [8].
Before the growth, the chamber was pumped down to a base pressure of, typically, less than 2:5 10-3 mtorr. SRO layers were deposited with argon sputtering; additional O2 was supplied during the deposition of SiO2 layers. The chamber pressures were 0.75 and 1.5 mtorr for the SRO and SiO2 depositions, respectively. Samples were grown without substrate heating and the RF power of 25 W was used. Under these conditions the growth rate was about 1 nm/min for both the SRO and SiO2 films which was determined by Dektak measurements by profiling areas with and without film. Samples were grown both on quartz and highly doped silicon substrates to facilitate optical and electrical characterisations, respectively. Substrates were cleaned with standard RCA1 and RCA2 solutions prior to the film deposition. Since samples on quartz and silicon were grown under same conditions, their compositional, optical and electrical properties are expected to be similar. Following the growth, the samples were typically annealed for 2 h at 1100 3 C in high purity nitrogen (99.9999). During the high temperature annealing, the excess Si in the SRO layer segregates to form QDs between the two oxide layers. The formation of QDs can be described with the following equation: SiOx -
x x SiO2 þ 1- Si 2 2
ð1Þ
This method of self-organised growth of Si quantum dots was first reported by Zacharias et al. [6] and has been developed at UNSW over the last few years using RF sputtering [7]. The size of the QDs is limited by the thickness of the SRO layer between the two barrier layers. Eq. (1) shows the dependence of density of quantum dots on the composition of the SRO layer. The density of the dots is higher if the SRO layer has a lower x, i.e., smaller O/Si ratio [6]. Several samples were deposited using different combinations of Si and quartz sputter targets to vary composition of SRO layers. In order to investigate size-controlled growth of Si QDs, several SiO2 =SRO=SiO2 structures with different SRO thicknesses were also grown using the same sputtering target. To facilitate comparison among different samples, oxide layer thicknesses were kept constant. Fig. 2 shows plan-view TEM image of a typical SiO2 =SRO=SiO2 structure grown on a silicon substrate after annealing at 11003 for 2 h. In this case a combined Si and quartz sputtering target with 33% silicon-excess, corresponding to oxygen to silicon ratio ¼ 1, was used. Silicon QDs in the oxide matrix are clearly visible. The size of the QDs is of about 5 nm which is similar to the thickness of the SRO layer, although slight variation in the QD sizes is evident.
Fig. 2. Typical TEM image of a SRO layer in SiO2 matrix after annealing at 1100 3 C. Si QDs are clearly visible.
Table 1 Oxygen to silicon ratio of silicon-rich oxide films as determined from sputter targets and Rutherford backscattering spectroscopy. Sample
ðO=SiÞTarget
ðO=SiÞRBS
A B C D
1.29 1.10 1.00 0.77
1.33 1.05 1.03 0.78
3. Results and discussion 3.1. Compositional analysis Rutherford backscattering spectroscopy (RBS) is a well established technique for the quantitative compositional depth profiling of thin films [9]. Thus, this technique has been used to investigate stoichiometry of SRO films obtained with different sputtering targets. SRO films were deposited using four different combined Si and quartz sputtering targets. The expected oxygento-silicon ratio, O/Si, for these films, as derived from the sputtering rates, are given in Table 1. RBS measurements were performed at the Australian National University, Canberra using 2 MeV 4He ion beam delivered by the 1.7 MV NEC Tandem accelerator. Projectile ions were detected at a scattering angle of y ¼ 1683 . Fig. 3 shows a measured energy spectrum from RBS measurement for a SRO film grown on a silicon substrate using 53% silicon-excess target (sample D). For each element, the high energy ions are from the surface of the sample, while the low energy ions are from deep inside the film. The silicon and oxygen from the film are clearly evident. The oxygen yield of the film, YO , was obtained by subtracting the oxygen signal from the background counts as shown in Fig. 3(b). The determination of silicon yield, YSi , was complicated since silicon was present both in the film and the substrate. The silicon in the film, in this case, was obtained by fitting the energy spectra with a combination of the linear and the error functions, as shown in Fig. 3(c). Here, the dashed-dot curve is the overall fit to the experimental data and the dashed curve is the contribution due to the substrate. The
ARTICLE IN PRESS 1548
S.K. Shrestha et al. / Solar Energy Materials & Solar Cells 94 (2010) 1546–1550
Fig. 3. (a) RBS energy spectrum from a typical SRO film. Fits to determine (b) oxygen and (c) silicon content of SRO film are shown.
solid curve shows the contribution due to the silicon from the film. It is evident from Fig. 3(b) and (c) that the quality of fits is very good. The quality of fit was similar for other samples. In order to compare the films from different growths, oxygen to silicon ratio of SRO films has been determined as follows: O YO YSi -1 ð2Þ ¼ dsO dsSi Si where dsO and dsSi are scattering cross section for oxygen and silicon, respectively. Results of RBS analysis are given in Table 1. The statistical uncertainty of the measurements was less than 1%. However, uncertainty of O/Si ratio derived from the fit may have been as high as 3% for few measurements. RBS measurements show that SiO2 is stoichiometric within the experimental uncertainties. Importantly, O/Si ratio of SRO films determined with RBS correlates well with that calculated from the sputtering targets. The discrepancy may be due to the uncertainty in the estimation of O/Si ratio from the sputtered targets, primarily due to different sputter rates for silicon and quartz targets. 3.2. Photoluminescence studies For the investigation of size-controlled growth of QDs, several samples consisting of a single layer Si QDs in SiO2 matrix were prepared by high temperature annealing of SiO2 =SRO=SiO2 layers. The SRO layer thicknesses were in the range of 1.8–7 nm. Thicknesses of the oxide layers were about 6 nm for all the samples, except for the two samples with SRO layer thickness of 1.8 nm and 2.4 nm. For these samples a 30 nm capping oxide layer was deposited to prevent possible oxidation of QD layer during
Fig. 4. (a) Results of PL measurement on a series of samples with a single layer of Si QDs in SiO2 matrix. (b) PL peak energy as a function of QD diameter. The solid curve is the best fit of PL energy with inverse square of QD diameter. For comparison data from selected literature are shown.
the high temperature annealing. For comparison, all the structures were grown with the same sputtering target (in this case 33% silicon-excess target with O=Si ¼ 1) and under similar growth conditions. Fig. 4(a) shows results of room temperature PL measurements on several samples. The 532 nm (2.34 eV) line of He-Cd laser with 50 mW power was used as the excitation source. PL signals were collected using a co-focal-lens system and detected with a double grating monochromator and an array of silicon photodetectors. The data were corrected for errors introduced by the collection system and spectral response of the detector array according to [10]. The normalised PL spectra from different samples are shown in the diagram. It should be noted that no distinct PL signature was observed from as-deposited samples. The signal-to-noise ratio is quite good, although PL was measured on a single layer of Si QDs, demonstrating a strong absorption coefficient of these QDs. This ratio is similar for the samples with SRO layer thickness greater than 3 nm, however, it is relatively weaker for the sample with smaller SRO layer thicknesses. Strong decrease in PL intensity from samples containing smaller nanocrystals are reported in literature which are attributed to decrease of the density of nanocrystals and the smaller absorption cross section [6,11]. These could also explain relatively weaker PL signal from our samples containing smaller QDs. The full width at half maximum of the PL peaks are quite broad ( 4 0:25 eV) suggesting a large size distribution of QDs in the samples. This is consistent with the TEM image where a slight variation in QD sizes has been observed (see Fig. 2). Large distributions of size of nanocrystals in multilayered structure or bulk film have also been reported, for example see [6,11]. However, PL peaks from different samples are clearly separated
ARTICLE IN PRESS S.K. Shrestha et al. / Solar Energy Materials & Solar Cells 94 (2010) 1546–1550
which demonstrates that average sizes of QDs are different for different samples. Importantly, a blue-shift of the PL peak is observed with the decrease in the SRO layer thickness. The shift is larger for the smaller QDs whereas it is smaller for the bigger QDs which are consistent with the quantum confinement model [12]. This is a significant result which shows that size of the quantum dots can be controlled with the SRO layer thickness. Fig. 4(b) shows the PL peak energy, derived from Fig. 4(a), as a function of the respective QD sizes. Here average QD size has been estimated to be equal to the SRO thicknesses, which is a reasonable estimation as the size of QDs is limited by the SRO layer thickness as discussed in Section 2. A non-linear decrease of PL energy with increasing QD sizes is evident. The decrease is rapid for the smaller QDs but slower for larger dots. The solid curve is the best fit of PL peak energy with inverse square of QD diameter which is in good agreement with the data. For comparison, data from selected literature [6,13,14] are shown which are in reasonable agreement with our data. Our data are also consistent with data for Si QDs in multilayered structure (SiO2 =Si QDs=SiO2 = . . . Si QDs=SiO2 ) and grown with similar technique [15]. This may indicate Si QDs in single and multilayered structures have similar optical properties.
3.3. Electrical measurements For I–V measurements to investigate resonant tunnelling through the double barrier structures discussed earlier, devices as schematically shown in the inset of Fig. 5 were fabricated using 33% silicon-excess target. This consisted of a single layer of Si QDs in SiO2 grown on a heavily doped Si substrate. A silicon capping layer of about 50 nm thickness was deposited with a highly doped silicon target. The thicknesses of the SRO and SiO2 layers were approximately 5 nm. A shadow mask was used during the growth to obtain isolated devices. The samples were annealed at 1100 3 C in nitrogen for 2 h during which QDs are formed. Aluminium contacts were deposited by evaporation using a shadow mask for electrical measurements. Fig. 5 shows results of room temperature I–V measurements on a typical device. Size of the mesa for this sample was about 5 mm2 . Here, the open and filled data points are the results of two subsequent measurements. In the both I–V profiles, an indication
1549
of negative differential resistance (NDR) can be observed around 1.3 V. Jiang et al. have described a model for resonant tunnelling through QDs in double barrier structure [16]. This model can, in principle, be applied to study the correlation of NDR peaks with resonant energy levels. However, more data would be required for reliable investigation. The observed NDR characteristics show increased conduction for the carriers in the energy range ( 1:321:5 eV) compared to the carriers outside this range. However, the observed NDR peak is very broad, which demonstrates a poor energy selectivity. Ideally hot carriers need to be extracted at a discrete energy through mono-energetic contacts for optimum efficiency [2]. However, in this case the power output from the device would be minimum. In a real device, however, width of the ESC, dE 40 which means that entropy will be generated on carrier cooling and thus efficiency will be reduced. Hence, dE should be kept as small as possible. A rough estimate of dE 25 meV has been recommended as a reasonable value for preliminary contact [5]. However, the observed half width of the NDR peak is more than 400 meV, which is an order of magnitude larger than the value suggested above. Hence, significant improvement in the quality of resonance is needed for these structures to be applicable as Energy Selective Contact for Hot Carrier solar cells. Nevertheless, such a result at room temperature is encouraging and demonstrates proof-of-concept. The broad NDR peak can be attributed to transport through QDs with slightly varying sizes around a mean diameter which will result in an overlap of resonant energy levels. This will be consistent with the broad PL peaks observed for all the samples suggesting a distribution of QD sizes in the structure. In addition, broadening of the NDR peak could be due to the deviation of QDs from the middle of the matrix which is expected to decrease the transmission probability significantly. It is also clear that the I–V profile has changed during the two subsequent measurements which indicate presence of defects in the material. Understanding of different types of defects in the SiO2 matrix and at the interfaces of QDs and SiO2 is a subject of further study. It should also be noted that device breakdown occurs if voltage in the access of, typically, 2 V is applied. The oxide strength of these samples is estimated to be about 1.3 MV/ cm which is much smaller than that for the thermal oxide. This may be primarily due to the presence of Si QDs and inferior quality of sputtered oxide. Further work is in progress to improve the quality of material, especially the oxide quality, for example, by growth at elevated temperature.
4. Conclusion
Fig. 5. Results of two consecutive I–V measurements at room temperature on a typical resonant tunnelling device are shown. The open and filled data points represent the first and the second measurements, respectively. Structure of the device is schematically shown in the inset.
Double barrier structures consisting of a single layer of Si QDs in SiO2 matrix have been studied for Energy Selective Contacts required for Hot Carrier solar cells. These structures were grown with RF magnetron sputtering using Si and quartz targets followed by high temperature annealing. The formation of quantum dots has been confirmed by transmission electron microscopy. Compositions of SRO films obtained with different sputtering targets have been accurately measured with Rutherford backscattering spectroscopy. Measurements show that O/Si ratio of SRO films correlates with the sputtering targets thus can be controlled. All the samples have shown good PL signals although they contained only a single layer of Si QDs. A blueshift of the PL peak is observed with the decrease in the SRO layer thickness which is consistent with quantum confinement model. Importantly, PL studies have demonstrated that the size of QDs can be controlled very well by the thickness of silicon-rich oxide layer, in between two oxide layers. I–V measurements have
ARTICLE IN PRESS 1550
S.K. Shrestha et al. / Solar Energy Materials & Solar Cells 94 (2010) 1546–1550
shown an evidence of NDR, which is a characteristic of resonant tunnelling, with some repeatability. However, the NDR peak is very broad suggesting a poor energy selectivity. Further work is focused on improving the quality of the material and position and quality of the resonance peak.
Acknowledgements The Photovoltaic Centre of Excellence is supported by the Australian Research Council. The authors would like to thank Shujuan Huang for the TEM images and Prof. Rob Elliman for the access to the RBS facility. References [1] R.T. Ross, A.J. Nozik, Efficiency of hot-carrier solar energy converters, J. Appl. Phys. 53 (1982) 3813–3818. ¨ [2] P. Wurfel, Solar energy conversion with hot electrons from impact ionisation, Sol. Energy Mater. Sol. Cells 46 (1997) 43–52. [3] M.A. Green, Third Generation Photovoltaics, Springer, Berlin, 2003. [4] Y. Takeda, et al., Hot carrier solar cells operating under practical conditions, J. Appl. Phys. 105 (2009) 074905:1–10.
[5] G.J. Conibeer, et al., Selective energy contacts for potential application to hot carrier PV cells, in: Proceedings of the 3rd World Conference on Photovoltaic Energy Conversion, Japan, 2003 pp. 2730–2733. [6] M. Zacharias, J. Heitmann, R. Scholz, U. Kahler, M. Schmidt, Size-controlled highly luminescent silicon nanocrystals: a SiO=SiO2 superlattice approach, J. Blasing Appl. Phys. Lett. 80 (2002) 661–663. [7] E.-C. Cho, et al., in: Proceedings of the 19th European Photovoltaic Solar Energy Conference, France, 2004, pp. 235–238. ¨ [8] P. Wurfel, A.S. Brown, T.E. Humphrey, M.A. Green, Particle conservation in the hot-carrier solar cell, Prog. Photovolt. Res. Appl. 13 (2005) 277–285. [9] L.C. Feldman, J.W. Mayer, Fundamentals of Surface and Thin Film Analysis, North-Holland, New York, 1986. [10] T.H. Gfroerer, Photoluminescence in analysis of surfaces and interfaces, Encyclopedia of Analytical Chemistry, 2000, pp. 9209–9231. [11] U. Kahler, H. Hofmeister, Visible light emission from Si nanocrystalline composites via reactive evaporation of SiO, Opt. Mater. 17 (2001) 83–86. [12] M.V. Wolkin, et al., Electronic states and luminescence in porous silicon quantum dots: the role of oxygen, Phys. Rev. Lett. 82 (1999) 197–200. [13] H. Takegi, et al., Quantum size effects on photoluminescence in ultrafine Si particles, Appl. Phys. Lett. 56 (1990) 2379–2380. [14] S. Takeoka, M. Fujii, S. Hayashi, Size-dependent photoluminescence from surface-oxidized Si nanocrystals in a weak confinement regime, Phys. Rev. B 62 (2000) 16820–16825. [15] G.J. Conibeer, et al., Silicon quantum dot nanostructures for tandem photovoltaic cells, Thin Solid Films 516 (2008) 6748–6756. [16] C.-W. Jiang, et al., Resonant tunneling through defects in an insulator: modeling and solar cell applications, J. Appl. Phys. 96 (2004) 5006–5012.